1. Field of the Invention
The present invention relates generally to an under-reaming pile bore excavator, and more particularly to an excavator such that an under-reamed part of a pile bore is excavated using a plurality of slidable wing bits moving downward and extending outward at the bottom of a straight pile bore along a plurality of guide rails fixed to a drill pipe at an appropriately inclined angle so as to cross the axis of the drill pipe in order to increase the diameter ratio of the under-reamed pile bore to the straight pile bore.
2. Description of the Prior Art
As an economical method of executing pile foundations, an under-reamed bore foundation is well known in which a larger diameter bore is excavated at the bottom of a straight pile bore to increase the end bearing capacity against a vertical load applied to the pile.
Conventionally, the method of reverse circulation drill excavation is used for executing these under-reamed pile foundations, in which the excavated soil is mixed by the rotational force applied to the main drill bits with water poured into the excavated pile bore and is pumped up to ground level through a bottom opening and the drill pipe.
In this method, however, since the under-reamed pile bore is excavated by extending the extendable wing bits horizontally by hydraulic cylinders without moving the wing bits downward, it is very difficult to form a stable impermeable surface layer preventing water from penetrating into the soil surface of the excavated bore and preventing lumps of soil from being dislodged therefrom, and additionally a relatively great torque is required to excavate the under-reamed part of the bore.
To overcome these problems in the prior-art under-reaming pile bore excavator, the inventor has already proposed a novel excavator disclosed under a patent application titled AN UNDER-REAMING PILE BORE EXCAVATOR AND METHOD OF ITS EXCAVATION. In the under-reaming pile bore excavator already proposed, however, it is difficult to excavate a larger-diameter under-reamed pile bore, compared with the straight pile bore, in order to further increase the end bearing capacity against a vertical load applied to the pile; in other words, it is difficult to increase the diameter ratio between the under-reamed pile bore and the straignt pile bore due to restrictions caused by its structure.